Light Pulses
Definition: flashes of light
More specific terms: Gaussian pulses, sech2-shaped pulses, parabolic pulses, solitons, quasi-soliton pulses, bandwidth-limited pulses, chirped pulses, double pulses, ultrashort pulses, laser pulses
German: Pulse
Author: Dr. Rüdiger Paschotta
Cite the article using its DOI: https://doi.org/10.61835/e9r
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Optical pulses are flashes of light. Particularly very short light pulses are often generated with lasers (laser pulses) and delivered in the form of laser beams, i.e., with a highly directional radiation.
Due to the enormously high optical frequencies, light pulses can be extremely short (ultrashort) when their optical bandwidth spans a significant fraction of the mean frequency. For example, a Gaussian pulse with a center frequency of 300 THz (corresponding to a wavelength of 1 μm) can easily have a bandwidth of 30 THz, and this already corresponds to a pulse duration of ≈ 15 fs if the pulse is transform-limited.
The shortest optical pulses generated directly in lasers (passively mode-locked titanium–sapphire lasers) have durations around 5 fs, corresponding only to a few optical cycles (few-cycle pulses). Pulse compression techniques applied to similar pulses reach pulse durations of very few femtoseconds, and high harmonic generation even allows the generation of attosecond pulses. On the other hand, many commercially important laser sources (particularly Q-switched lasers) generate nanosecond pulses (often with considerable pulse energies), which are considered as short but not ultrashort. Nanosecond pulses (from nanosecond lasers) also have many important applications, e.g. in laser material processing.
Depending on the required pulse duration, pulse energy, and pulse repetition rate, different methods of pulse generation, pulse compression and pulse characterization are used, overall covering extremely wide parameter regimes. See the corresponding articles for details, in particular the one on pulse characterization.
High Peak Powers and Intensities
Due to the short pulse durations and the potential for strong focusing, optical pulses can be used for generating extremely high optical intensities even with moderate pulse energies. For example, a 10-fs pulse with only 10 mJ energy has a peak power of the order of 1 TW = 1000 GW, corresponding to the combined power of roughly 1000 large nuclear power stations. This power may be easily focused to spots with a diameter of only a few micrometers. Therefore, amplified ultrashort pulses are very important for high-intensity physics, studying phenomena such as multi-photon ionization, high harmonic generation, or the generation of even shorter pulses with attosecond durations.
Characterization of Light Pulses
There are various methods for measuring the pulse duration achieved or for pulse characterization in other respects. Particularly for measuring the duration of ultrashort pulses, purely optical techniques are very important, since electronics are too slow for such purposes.
Single Shot or Repetitive Pulse Generation
Short laser pulses in the nanosecond pulse duration regime are often generated either in a single-shot regime (pulse on demand, with long and potentially irregular breaks between the pulses) or in a repetitive mode with a pulse repetition rate which is often in the kilohertz region. In contrast to that, ultrashort pulses (i.e., with durations in the picosecond or femtosecond region) are often generated as pulse trains with high repetition frequencies of many megahertz or even many gigahertz.
Bursts of Pulses
In some cases, laser sources do not generate a periodic sequence of pulses, but rather a periodic sequence of pulse bursts, where each burst consists of some number of short or ultrashort pulses. Within the larger a burst, one may have a high pulse repetition rate e.g. in the megahertz or gigahertz region, while the repetition rate of the bursts can be much lower, e.g. in the kilohertz region or even less.
For more details, see the article on burst mode lasers.
Pulse Propagation
Pulse propagation in media has many interesting aspects. The peak of a pulse in a transparent medium propagates with the group velocity, not the phase velocity. Dispersion can cause temporal broadening (or sometimes compression) of pulses. For high peak intensities, optical nonlinearities can strongly affect the pulse propagation; often they lead to pulse broadening, but strong nonlinear compression is also possible.
Apart from experimental tests, details of pulse propagation can also be investigated with various kinds of numerical simulation. In some cases, for example for pulse propagation in single-mode fibers or free-space propagation with a fixed Gaussian beam profile, one can neglect the transverse spatial dimensions and consider only the complex amplitude versus time or frequency at each location. More sophisticated numerical models are required for investigating the full spatio–temporal pulse evolution.
Tutorials and Case Studies
See our tutorials Modeling of Fiber Amplifiers and Lasers.
The following case studies are available, which discusses some aspects of pulse propagation modeling:
- Pulse compression in a fiber
- We explore how we can spectrally broaden light pulses by self-phase modulation in a fiber and subsequently compress the pulses using a dispersive element. A substantial reduction in pulse duration by more than an order of magnitude is easily achieved, while the pulse quality is often not ideal.
- Collision of soliton pulses in a fiber
- We let two soliton pulses collide in a fiber. Surprisingly, they survive such collisions, even if we involve solitons of higher order.
- Solitons in a fiber amplifier
- We investigate to which extent soliton pulses could be amplified in a fiber amplifier, preserving the soliton shape and compressing the pulses temporally.
- Parabolic pulses in a fiber amplifier
- We explore the regime of parabolic pulse amplification in an Yb-doped single-mode fiber. We find reasonable operation parameters and investigate various kinds of limitations, e.g. concerning the nonlinear pulse compression.
- Erbium-doped fiber amplifier for rectangular nanosecond pulses
- Specifically, we deal with deformations of the pulse shape due to gain saturation. These can be minimized by pre-distorting the input pulses.
- Raman scattering in a fiber amplifier
- We investigate the effects of stimulated Raman scattering in an ytterbium-doped fiber amplifier, considering three very different input pulse duration regimes. Surprisingly, the effect of Raman scattering always gets substantial only on the last meter, although the input peak powers vary by two orders of magnitude.
Bibliography
[1] | R. Paschotta, Field Guide to Laser Pulse Generation, SPIE Press, Bellingham, WA (2007) |
See also: pulse trains, ultrashort pulses, double pulses, pulse duration, pulse energy, pulse repetition rate, carrier–envelope offset, spectral phase, pulse generation, pulse characterization, pulse propagation modeling, pulsed lasers, burst mode lasers, ultrafast lasers, mode-locked lasers, Q-switched lasers
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